This invention relates to a method of extrusion, and to an improved extrusion press, to make the extrusion process more efficient and less wasteful of the material being extruded.
Although the present invention will be particularly described with reference to thee extrusion process as applied to aluminum alloys, the invention can be applied to other materials, which are pressure-extrudable.
An extrusion press is a machine that is designed to apply a known and controlled directional force, of known magnitude, against a known orifice via a quantity of aluminum alloy physically contained. The aluminum alloy is contained during the application of the applied force by a component forming part of the machine and known as the container.
The purpose of the container is to allow the force applied to the aluminum alloy contained within the internal dimensions or bore of the container to be converted into linear movement in the direction of the applied force through a component containing one or more precision orifices or dies constituting a die plate.
During the time that the aluminum alloy is subjected to linear movement and pressure the container is physically clamped to the die plate.
An extrusion die plate is a means of providing physical resistance to the linear movement of the contained aluminum alloy in order to allow the aluminum alloy to be shaped by the detailed geometry of the orifice. The extrusion die plate is manufactured with one or more precision orifices designed to allow the aluminum alloy that passes through the orifice or orifices to assume the required product shape.
The temperature of the aluminum alloy is raised to a known value prior to the application of the applied force.
The action of applying sufficient force to an aluminum alloy under these conditions results in the aluminum alloy being physically forced through the orifice, to produce a product of cross-section conforming to the shape of the orifice.
The method of generating the directional force, known as the extrusion pressure, is in principle that of a hydraulic ram and cylinder.
The extrusion pressure generated by the principal or main ram of the extrusion press machine is transmitted to the aluminum alloy contained in the container in a linear manner by means of a steel rod attached to the main ram via the ram crosshead and known as the extrusion stem.
A pressure pad is employed between the extrusion stem and the contained aluminum alloy. The pressure pad is designed to transmit the extrusion pressure from the extrusion stem to the contained aluminum alloy while permitting an aluminum alloy film of approximately 0.4 mm thick to be left in the container bore after the pressure pad has passed through the container under extrusion pressure. The consolidated aluminum remaining in the container after extrusion is known as the "discard".
The process of preparing an extrusion press to extrude a quantity of aluminum alloy often results in entrapping unwanted air in the cavities Within the container prior to the aluminum being subjected to the extrusion pressures.
If the entrapped air is pressurized by the extrusion process the results can be seen as severe damage to the finish of the extruded product, in the form of blistering, often accompanied by audible sound of the gases upon release to the atmosphere after passing through the die orifice.
The sequence of events relating to the physical movements of an extrusion press to perform the function of extruding hot aluminum alloy through an extrusion die can be broken down into specific stages known as press cycles.
A standard press cycle for the extrusion process in order to eliminate the problem of entrapped air is a cycle called the de-gas or "burp" cycle.
The de-gas cycle requires the aluminum alloy to be consolidated in the container at approximately half of the required extrusion pressure. The de-gas cycle then decompresses the components that are normally under extrusion or clamping pressure as in the case of the main ram, container and die plate.
The components are physically moved to break the seal around the area of the extrusion die to permit pressurized gases to escape. The de-gas cycle is then completed by re-initiating the container "close and seal" cycle as well as returning the main ram to its previous extrusion condition.
It has already been proposed, in UK Patent Specification No. GB-A-1,462,163, to remove air from the void volume within a billet container by applying a source of vacuum to an opening in the sidewall of the container. However, this method of evacuation will tend to draw air past the ram and will not work in evacuating the void space until the ram has consolidated the billet, at which point the opening will have been closed off by the ram.